The present invention relates to a spin transfer torque magnetic random access memory (STT-MRAM) device, and more particularly, to a memory element of the STT-MRAM device including a dielectric layer for generating electric field to assist switching.
Spin transfer torque magnetic random access memory (STT-MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An STT-MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection element coupled in series between appropriate electrodes. Upon application of an appropriate voltage or current to the magnetic memory element, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
FIG. 1 shows a conventional memory element for an STT-MRAM device comprising a magnetic reference layer 12 and a magnetic free layer 14 with an insulating tunnel junction layer 16 interposed therebetween, thereby collectively forming a magnetic tunneling junction (MTJ) 18. The magnetic reference layer 12 and free layer 14 have magnetization directions 20 and 22, respectively, which are substantially perpendicular to the layer plane. Therefore, the MTJ 18 is a perpendicular type comprising the magnetic layers 12 and 14 with perpendicular anisotropy. Upon application of a switching current through the perpendicular MTJ 18, the magnetization direction 22 of the magnetic free layer 14 can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction 20 of the magnetic reference layer 12. The insulating tunnel junction layer 16 is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. However, when the magnetization directions 22 and 20 of the magnetic free layer 14 and reference layer 12 are substantially parallel, electrons polarized by the magnetic reference layer 12 can tunnel through the insulating tunnel junction layer 16, thereby decreasing the electrical resistivity of the perpendicular MTJ 18. Conversely, the electrical resistivity of the perpendicular MTJ 18 is high when the magnetization directions 20 and 22 of the magnetic reference layer 12 and free layer 14 are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction 22 of the magnetic free layer 14.
A recent study by Wang et al. on perpendicular MTJ shows that the perpendicular anisotropy of magnetic layers in magnesium oxide (MgO) based MTJ structures can be changed by the voltage applied to the magnetic layers. See Wei-Gang Wang et al., “Electric-field-assisted switching in magnetic tunnel junctions,” Nature Materials Vol. 11, 64-68 (2012).
In the test set-up of Wang et al. as shown in FIG. 1, a positive electric potential is applied to the MTJ 18 to drive electrons into the magnetic free layer 14 made of 1.6 nm thick Co40Fe40B20. When the insulating tunnel junction layer 16, which is made of 1.4 nm thick magnesium oxide (MgO), is sufficiently thick and electrical resistance across the MgO junction layer 16 is sufficiently high, the current density through the MgO junction layer 16 will be low. In this case, the magnetic reference layer 12, which is made of 1.3 nm thick Co40Fe40B20, and the magnetic free layer 14 adjacent to the MgO junction layer 16 effectively form a parallel plate capacitor with the MgO junction layer 16 acts as the dielectric. When a voltage is applied to the MTJ 18, electrical charges will accumulate in the two magnetic layers 12 and 14 like a capacitor, resulting in formation of an electric field 24 across the MgO junction layer 16. The applied positive voltage as shown in FIG. 1 causes the magnetic free layer 14 to have a negative potential, i.e. electron accumulation at the interface between the magnetic free layer 14 and the MgO junction layer 16. With increasing applied voltage and electron accumulation at the interface, the magnetic free layer 14 shows decreasing perpendicular anisotropy, which is reflected by decreasing coercivity field Hc. The decreasing of perpendicular anisotropy and corresponding coercivity field would facilitate the switching of the variable magnetization direction 22 of the magnetic free layer 14 from parallel to anti-parallel orientation. In contrast, electrons will be depleted at the interface between the magnetic reference layer 12 and the MgO junction layer 16 with increasing applied voltage, resulting in increasing perpendicular anisotropy and coercivity field for the magnetic reference layer 12. It should be noted that Wang's finding can only be used to help switching the variable magnetic moment 22 of the magnetic free layer 14 from parallel to anti-parallel orientation, not the other way, i.e. anti-parallel to parallel orientation.
A possible explanation of the observed changes in perpendicular anisotropy with electron accumulation/depletion at interfaces between magnetic free layer/MgO junction layer/magnetic reference layer as reported by Wang et al. may be that having increased amount of electrons at the interface between the magnetic free layer 14 and the MgO junction layer 16 allows more conductive electrons to fill the 3d-band of CoFe lattice and reduce the unpaired 3d-valence electron population, thereby making the broken-symmetry induced surface perpendicular anisotropy weaker and the magnetic free layer 14 magnetically softer. When 3d-electrons are depleted at the interface between the magnetic reference layer 12 and the MgO junction layer 16, electrons will be depleted first from paired 3d-electrons according to Hunt's Rules. As more 3d-electrons become unpaired in the magnetic reference layer 12, the surface perpendicular anisotropy thereof increases, making the magnetic reference layer 12 magnetically harder to switch by external field or spin transfer torque.
While a conventional MTJ having an MgO junction layer can exhibit the above-described electric field assisted switching effect, the effect is not significant because the MgO junction layer is thin enough to allow a relatively high density of electrons to tunnel therethrough, thereby minimizing the capacitive effect needed to generate electrons at the interface between the magnetic layer and the MgO junction layer. Increasing the MgO thickness can improve the capacitive effect but would also adversely increase the MTJ resistance. Moreover, the prior art method illustrated in FIG. 1 can only help switching a magnetic free layer from parallel to anti-parallel orientation, i.e. low resistance to high resistance state.
For the foregoing reasons, there is a need for an STT-MRAM device having MTJ memory elements that can be easily switched and a method for switching the memory elements between low and high resistance state.